scholarly journals An Idealized Numerical Simulation Investigation of the Effects of Surface Drag on the Development of Near-Surface Vertical Vorticity in Supercell Thunderstorms

2016 ◽  
Vol 73 (11) ◽  
pp. 4349-4385 ◽  
Author(s):  
Paul M. Markowski
Keyword(s):  
Lower Boundary ◽  
Cold Pool ◽  
Surface Drag ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Supercell Storms ◽  
Scale Turbulence ◽  
Barotropic Vorticity ◽  
Pool Formation

Abstract Idealized simulations are used to investigate the contributions of frictionally generated horizontal vorticity to the development of near-surface vertical vorticity in supercell storms. Of interest is the relative importance of barotropic vorticity (vorticity present in the prestorm environment), baroclinic vorticity (vorticity that is principally generated by horizontal buoyancy gradients), and viscous vorticity (vorticity that originates from the subgrid-scale turbulence parameterization, wherein the effects of surface drag reside), all of which can be advected, tilted, and stretched. Equations for the three partial vorticities are integrated in parallel with the model. The partial vorticity calculations are complemented by analyses of circulation following material circuits, which are often able to be carried out further in time because they are less susceptible to explosive error growth. Near-surface mesocyclones that develop prior to cold-pool formation (this only happens when the environmental vorticity is crosswise near the surface) are dominated by only barotropic vertical vorticity when the lower boundary is free slip, but both barotropic and viscous vertical vorticity when surface drag is included. Baroclinic vertical vorticity grows large once a cold pool is established, regardless of the lower boundary condition and, in fact, dominates at the time the vortices are most intense in all but one simulation (a simulation dominated early by a barotropic mode of vortex genesis that may not be relevant to real convective storms).

scholarly journals Dynamics of Lower-Tropospheric Vorticity in Idealized Simulations of Tropical Cyclone Formation

2019 ◽  
Vol 76 (3) ◽  
pp. 707-727 ◽  
Author(s):  
Yaping Wang ◽  
Christopher A. Davis ◽  
Yongjie Huang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mass Flux ◽  
Cloud Model ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Cold Pool ◽  
Surface Drag ◽  
Near Surface ◽  
Cold Pools

Abstract Idealized simulations are conducted using the Cloud Model version 1 (CM1) to explore the mechanism of tropical cyclone (TC) genesis from a preexisting midtropospheric vortex that forms in radiative–convective equilibrium. With lower-tropospheric air approaching near saturation during TC genesis, convective cells become stronger, along with the intensifying updrafts and downdrafts and the larger area coverage of updrafts relative to downdrafts. Consequently, the low-level vertical mass flux increases, inducing vorticity amplification above the boundary layer. Of interest is that while surface cold pools help organize lower-tropospheric updrafts, genesis still proceeds, only slightly delayed, if subcloud evaporation cooling and cold pool intensity are drastically reduced. More detrimental is the disruption of near saturation through the introduction of weak vertical wind shear. The lower-tropospheric dry air suppresses the strengthening of convection, leading to weaker upward mass flux and much slower near-surface vortex spinup. We also find that surface spinup is similarly inhibited by decreasing surface drag despite the existence of a nearly saturated column, whereas larger drag accelerates spinup. Increased vorticity above the boundary layer is followed by the emergence of a horizontal pressure gradient through the depth of the boundary layer. Then the corresponding convergence resulting from the gradient imbalance in the frictional boundary layer causes vorticity amplification near the surface. It is suggested that near saturation in the lower troposphere is critical for increasing the mass flux and vorticity just above the boundary layer, but it is necessary yet insufficient because the spinup is strongly governed by boundary layer dynamics.

scholarly journals Interannual Variability of Deep-Layer Hydrologic Memory and Mechanisms of Its Influence on Surface Energy Fluxes

2005 ◽  
Vol 18 (23) ◽  
pp. 5024-5045 ◽  
Author(s):  
Geremew G. Amenu ◽  
Praveen Kumar ◽  
Xin-Zhong Liang
Keyword(s):  
Soil Moisture ◽  
Soil Temperature ◽  
Annual Cycle ◽  
Temporal Variability ◽  
Deep Layer ◽  
Lower Boundary ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Lower Boundary Condition ◽  
Near Surface

Abstract The characteristics of deep-layer terrestrial memory are explored using observed soil moisture data and simulated soil temperature from the Illinois Climate Network stations. Both soil moisture and soil temperature are characterized by exponential decay in amplitude, linear lag in phase, and increasing persistence with depth. Using spectral analysis, four dominant low-frequency modes are identified in the soil moisture variability. These signals have periods of about 12, 17, 34, and 60 months, which correspond to annual cycle, (4/3) ENSO, quasi-biennial (QB) ENSO, and quasi-quadrennial (QQ) ENSO signals, respectively. For deep layers, the interannual modes are dominant over the annual cycle, and vice versa for the near-surface layer. There are inherently two mechanisms by which deep-layer moisture impacts the surface fluxes. First, its temporal variability sets the lower boundary condition for the transfer of moisture and heat fluxes from the surface. Second, this temporal variability influences the uptake of moisture by plant roots, resulting in the variability of the transpiration and, therefore, the entire energy balance. Initial results suggest that this second mechanism may be more predominant.

scholarly journals A Unifying Perspective on Transfer Function Solutions to the Unsteady Ekman Problem

Fluids ◽  
2021 ◽  
Vol 6 (2) ◽  
pp. 85
Author(s):  
Jonathan M. Lilly ◽  
Shane Elipot
Keyword(s):  
Transfer Function ◽  
Impulse Response Function ◽  
Lower Boundary ◽  
Finite Depth ◽  
Systematic Investigation ◽  
Numerical Code ◽  
Uniform Motion ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
The Fourier Transform

The unsteady Ekman problem involves finding the response of the near-surface currents to wind stress forcing under linear dynamics. Its solution can be conveniently framed in the frequency domain in terms of a quantity that is known as the transfer function, the Fourier transform of the impulse response function. In this paper, a theoretical investigation of a fairly general transfer function form is undertaken with the goal of paving the way for future observational studies. Building on earlier work, we consider in detail the transfer function arising from a linearly-varying profile of the vertical eddy viscosity, subject to a no-slip lower boundary condition at a finite depth. The horizontal momentum equations, rendered linear by the assumption of horizontally uniform motion, are shown to transform to a modified Bessel’s equation for the transfer function. Two self-similarities, or rescalings that each effectively eliminate one independent variable, are identified, enabling the dependence of the transfer function on its parameters to be more readily assessed. A systematic investigation of asymptotic behaviors of the transfer function is then undertaken, yielding expressions appropriate for eighteen different regimes, and unifying the results from numerous earlier studies. A solution to a numerical overflow problem that arises in the computation of the transfer function is also found. All numerical code associated with this paper is distributed freely for use by the community.

scholarly journals The Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercell Thunderstorms

2019 ◽  
Vol 76 (5) ◽  
pp. 1349-1372 ◽  
Author(s):  
Matthew Brown ◽  
Christopher J. Nowotarski
Keyword(s):  
Necessary Condition ◽  
Cold Pool ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Low Level ◽  
Cold Pools ◽  
Surface Circulation ◽  
Supercell Thunderstorms ◽  
Wind Profiles ◽  
Lifting Condensation Level

Abstract This paper reports on results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the morphology and evolution of low-level rotation in supercell thunderstorms. Previous studies have shown that the LCL can influence outflow buoyancy, which can in turn affect generation and stretching of near-surface vertical vorticity. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles with varying LCLs are implemented and compared over a variety of low-level wind profiles. The thermodynamic properties of the simulations are sensitive to variations in the LCL, with higher LCLs contributing to more negatively buoyant cold pools. These outflow characteristics allow for a more forward propagation of near-surface circulation relative to the midlevel mesocyclone. When the mid- and low-level mesocyclones become aligned with appreciable near-surface circulation, favorable dynamic updraft forcing is able to stretch and intensify this rotation. The strength of the vertical vorticity generated ultimately depends on other interrelated factors, including the amount of near-surface circulation generated within the cold pool and the buoyancy of storm outflow. However, these simulations suggest that mesocyclone alignment with near-surface circulation is modulated by the ambient LCL, and is a necessary condition for the strengthening of near-surface vertical vorticity. This alignment is also sensitive to the low-level wind profile, meaning that the LCL most favorable for the formation of intense vorticity may change based on ambient low-level shear properties.

scholarly journals LES of Laminar Flow in the PBL: A Potential Problem for Convective Storm Simulations

2016 ◽  
Vol 144 (5) ◽  
pp. 1841-1850 ◽  
Author(s):  
Paul M. Markowski ◽  
George H. Bryan
Keyword(s):  
Lower Boundary ◽  
Vertical Wind Shear ◽  
Surface Friction ◽  
Vertical Wind ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
Severe Storms ◽  
Turbulent Eddies ◽  
Wind Profiles ◽  
Tornadic Storms

In idealized simulations of convective storms, which are almost always run as large-eddy simulations (LES), the planetary boundary layers (PBLs) are typically laminar (i.e., they lack turbulent eddies). When compared with turbulent simulations, theory, or simulations with PBL schemes, the typically laminar LES used in the severe-storms community produce unrealistic near-surface vertical wind profiles containing excessive vertical wind shear when the lower boundary condition is nonfree slip. Such simulations are potentially problematic given the recent interest within the severe storms community in the influence of friction on vorticity generation within tornadic storms. Simulations run as LES that include surface friction but lack well-resolved turbulent eddies thus probably overestimate friction’s effects on storms.

scholarly journals Low-level Updraft Intensification in Response to Environmental Wind Profiles

2021 ◽  
Author(s):  
Nicholas A. Goldacker ◽  
Matthew D. Parker
Keyword(s):  
Lower Troposphere ◽  
Dynamical Response ◽  
Streamwise Vorticity ◽  
Self Organizing Maps ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Low Level ◽  
Supercell Storms ◽  
Order Of Magnitude ◽  
Wind Profiles

AbstractSupercell storms can develop a “dynamical response” whereby upward accelerations in the lower troposphere amplify as a result of rotationally induced pressure falls aloft. These upward accelerations likely modulate a supercell’s ability to stretch near-surface vertical vorticity to achieve tornadogenesis. This study quantifies such a dynamical response as a function of environmental wind profiles commonly found near supercells. Self-organizing maps (SOMs) were used to identify recurring low-level wind profile patterns from 20,194 model-analyzed, near-supercell soundings. The SOM nodes with larger 0–500 m storm-relative helicity (SRH) and streamwise vorticity (ωs) corresponded to higher observed tornado probabilities. The distilled wind profiles from the SOMs were used to initialize idealized numerical simulations of updrafts. In environments with large 0–500 m SRH and large ωs, a rotationally induced pressure deficit, increased dynamic lifting, and a strengthened updraft resulted. The resulting upward-directed accelerations were an order of magnitude stronger than typical buoyant accelerations. At 500 m AGL, this dynamical response increased the vertical velocity by up to 25 m s–1, vertical vorticity by up to 0.2 s–1, and pressure deficit by up to 5 hPa. This response specifically augments the near-ground updraft (the midlevel updraft properties are almost identical across the simulations). However, dynamical responses only occurred in environments where 0–500 m SRH and ωs exceeded 110 m2 s–2 and 0.015 s–1, respectively. The presence vs. absence of this dynamical response may explain why environments with higher 0–500 m SRH and ωs correspond to greater tornado probabilities.

scholarly journals Near-Ground Rotation in Simulated Supercells: On the Robustness of the Baroclinic Mechanism*

2015 ◽  
Vol 143 (12) ◽  
pp. 4929-4942 ◽  
Author(s):  
Johannes M. L. Dahl
Keyword(s):  
Trajectory Analysis ◽  
Relative Vorticity ◽  
Cyclonic Vorticity ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Surface Rotation ◽  
Straight Line ◽  
Single Moment ◽  
Double Moment ◽  
Barotropic Vorticity

Abstract This study addresses the robustness of the baroclinic mechanism that facilitates the onset of surface rotation in supercells by using two idealized simulations with different microphysics parameterizations and by considering previous results. In particular, the importance of ambient crosswise vorticity relative to baroclinically generated vorticity in the development of near-ground cyclonic vorticity is analyzed. The storms were simulated using the CM1 model in a kinematic base state characterized by a straight-line hodograph. A trajectory analysis spanning about 30 min was performed for a large number of parcels that contribute to near-surface vertical-vorticity maxima. The vorticity along these trajectories was decomposed into barotropic and nonbarotropic parts, where the barotropic vorticity represents the effects of the preexisting, substantially crosswise horizontal storm-relative vorticity. The nonbarotropic part represents the vorticity produced baroclinically within the storm. It was found that the imported barotropic vorticity attains a downward component near the surface, while the baroclinic vorticity points upward and dominates. This dominance of the baroclinic vorticity is independent of whether a single-moment or double-moment microphysics parameterization is used. A scaling argument is offered as explanation, predicting that the baroclinic vertical vorticity becomes increasingly dominant as downdraft strength increases.

scholarly journals Significance of a Midlatitude SST Frontal Zone in the Formation of a Storm Track and an Eddy-Driven Westerly Jet*

2010 ◽  
Vol 23 (7) ◽  
pp. 1793-1814 ◽  
Author(s):  
Takeaki Sampe ◽  
Hisashi Nakamura ◽  
Atsushi Goto ◽  
Wataru Ohfuchi
Keyword(s):  
General Circulation ◽  
Frontal Zone ◽  
Sensible Heat Flux ◽  
Lower Boundary ◽  
Storm Track ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
Low Level ◽  
Sst Front ◽  
Sst Gradient

Abstract In a set of idealized “aquaplanet” experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a storm track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another storm track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observed, consistent with westerly momentum transport associated with baroclinic eddy growth. The sharp poleward decline in the surface sensible heat flux across the SST frontal zone sustains strong near-surface baroclinicity against the relaxing effect by vigorous poleward eddy heat transport. Elimination of the midlatitude frontal SST gradient yields marked decreases in the activity of eddies and their transport of angular momentum into midlatitudes, in association with equatorward shifts of the PFJ-associated low-level westerlies and a subtropical high pressure belt, especially in the summer hemisphere. These impacts of the midlatitude frontal SST gradient are found to be robust against modest changes in the STJ intensity as observed in its interannual variability, suggesting the potential importance of midlatitude atmosphere–ocean interaction in shaping the tropospheric general circulation.

Observations of Near-Surface Vertical Wind Profiles and Vertical Momentum Fluxes from VORTEX-SE 2017: Comparisons to Monin–Obukhov Similarity Theory

2019 ◽  
Vol 147 (10) ◽  
pp. 3811-3824 ◽  
Author(s):  
Paul M. Markowski ◽  
Nathan T. Lis ◽  
David D. Turner ◽  
Temple R. Lee ◽  
Michael S. Buban
Keyword(s):  
Boundary Condition ◽  
Similarity Theory ◽  
Lower Boundary ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
Convective Storms ◽  
Momentum Fluxes ◽  
Wind Profiles

Abstract Observations of near-surface vertical wind profiles and vertical momentum fluxes obtained from a Doppler lidar and instrumented towers deployed during VORTEX-SE in the spring of 2017 are analyzed. In particular, departures from the predictions of Monin–Obukhov similarity theory (MOST) are documented on thunderstorm days, both in the warm air masses ahead of storms and within the cool outflow of storms, where MOST assumptions (e.g., horizontal homogeneity and a steady state) are least credible. In these regions, it is found that the nondimensional vertical wind shear near the surface commonly exceeds predictions by MOST. The departures from MOST have implications for the specification of the lower boundary condition in numerical simulations of convective storms. Documenting departures from MOST is a necessary first-step toward improving the lower boundary condition and parameterization of near-surface turbulence (“wall models”) in storm simulations.

Tornadogenesis in a High-Resolution Simulation of the 8 May 2003 Oklahoma City Supercell

2013 ◽  
Vol 71 (1) ◽  
pp. 130-154 ◽  
Author(s):  
Alexander D. Schenkman ◽  
Ming Xue ◽  
Ming Hu
Keyword(s):  
Dominant Role ◽  
Oklahoma City ◽  
Prediction System ◽  
Grid Spacing ◽  
Surface Drag ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Advanced Regional Prediction System ◽  
Regional Prediction ◽  
First Time

Abstract A 50-m-grid-spacing Advanced Regional Prediction System (ARPS) simulation of the 8 May 2003 Oklahoma City tornadic supercell is examined. A 40-min forecast run on the 50-m grid produces two F3-intensity tornadoes that track within 10 km of the location of the observed long-track F4-intensity tornado. The development of both simulated tornadoes is analyzed to determine the processes responsible for tornadogenesis. Trajectory-based analyses of vorticity components and their time evolution reveal that tilting of low-level frictionally generated horizontal vorticity plays a dominant role in the development of vertical vorticity near the ground. This result represents the first time that such a mechanism has been shown to be important for generating near-surface vertical vorticity leading to tornadogenesis. A sensitivity simulation run with surface drag turned off was found to be considerably different from the simulation with drag included. A tornado still developed in the no-drag simulation, but it was much shorter lived and took a substantially different track than the observed tornadoes as well as the simulated tornadoes in the drag simulation. Tilting of baroclinic vorticity in an outflow surge may have played a role in tornadogenesis in the no-drag simulation.

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